Properties required for a grease composition to increase its lubrication life at a high temperature are low evaporation loss, high oxidation stability, excellent antirust property, excellent lubricating property and the like. There have been various proposals to improve antirust property and lubrication life of a grease composition.
For example, there has been proposed a grease composition based on perfluoropolyether or fluoropolyether derivatives, which shows excellent performance in applications where the parts to be lubricated run at high speeds, at the same time exhibiting the good performance of greases based on non-functional perfluoropolyethers as regards the viscostaticity as a function of temperature, the vapor tension and the lubricating properties under limit conditions. The grease composition also shows high anticorrosive property (see Patent Document 1).
There has also been proposed a lubricating grease composition which comprises a perfluoropolyether base oil and as a thickening agent, at least one of an aliphatic dicarboxylic acid metal salt, a monoamide monocarboxylic acid metal salt, and a monoester carboxylic acid metal salt, and the grease composition shows antiwear property to a material to be lubricated, antileakage property, and washing property and it is inexpensive (see Patent Document 2).
In order to impart antirust property to a grease composition, it is general to add an antirust agent. An inorganic passivating agent is usually used as an antirust agent in a fluorine containing grease composition for a ball bearing used at a high temperature. In particular, sodium nitrite has been widely used as most effective antirust agent. However, it is known that sodium nitrite reacts with a secondary amine under acidic condition to form N-nitrosamine. Since N-nitrosamine is an environmentally harmful substance, there is a need for an antirust agent in place of sodium nitrite.
[Patent Document 1] JP-A-1-272696
[Patent Document 2] JP-A-2001-354986